1. Field of the Invention
The present invention relates to the constitution of a welded portion between a plurality of members, and a welding method.
2. Description of the Related Art
Welding has been used conventionally on various manufacturing sites as a means of joining metallic members to each other. For example, welding is performed in the manufacturing process of a vehicle axle case.
An axle case accommodates the driving shaft, differential gear, and so on of a vehicle such as a truck, and as shown in FIG. 10, comprises a main body 38 extending in the vehicle width direction of the vehicle and constituted by an upper member 30 and a lower member 31 which are joined by being laid one above the other such that the two end portions in the longitudinal direction are formed into a substantially cylindrical form, and a substantially cylindrical spindle 33 which is joined to the two end portions in the longitudinal direction of the main body 38. The upper and lower members 30, 31 are curved upward or downward at the central portion thereof in the longitudinal direction to form a substantially circular hole (not shown in the drawing) in the central portion in the longitudinal direction of the main body 38. A hemispherical cover member 32 is mounted on the main body 38 so as to cover the hole. An axle case typically has an assembled sheet metal structure, and similar axle cases are described in Japanese Patent Application Publication H08-067108 and so on, for example.
In this type of axle case 34, the two end portions in the longitudinal direction of the main body 38 and the spindle 33 are joined by butt welding. More specifically, as shown in FIG. 11, an end portion of the main body 38 and an end portion of the spindle 33 are caused to abut and then joined by welding. Note that FIG. 11 shows a cross section of an upper portion joint portion between the main body 38 and spindle 33.
Further, the cover member 32 and the upper and lower members 30, 31 are joined by being laid together and fillet welded. More specifically, as shown in FIG. 12, a flange 32a formed on the peripheral edge portion of the cover member 32 is superposed on the upper and lower members 30, 31 and joined thereto by fillet welding.
Incidentally, various loads and moments act on the axle case 34 as a result of acceleration, deceleration, and changes of direction in the vehicle, irregularities on the surface of the road along which the vehicle is traveling, and so on. In particular, moments often act to bend the axle case 34 in an upward or downward direction. For example, a driving wheel is provided on the outer peripheral portion of the spindle 33, and hence when an upward reactive force is received from the road surface, a moment M (see FIG. 10) acts to raise the two longitudinal direction end portions which serve as supports for the substantial central portion of the axle case 34 in the longitudinal direction.
As a result, stress is generated on the joint portion between the main body 38 and the spindle 33.
Accordingly, a comparatively high degree of strength is required in the welded portion between the main body 38 and spindle 33. To achieve this, conventionally the penetration ratio of the joint portion between the main body 38 and spindle 33 in relation to the sheet thickness of the main body 38 and spindle 33 has been set at 100%.
If the penetration ratio is set at 100% when butt welding a sheet material, welding is usually performed from both the front and rear sides of the sheet material. However, the spindle 33 takes a form in which the outside end portion thereof has a reduced diameter, and it is therefore difficult to insert a welding rod or the like into the interior and to perform welding from the rear. If welding is performed only from the front side with a large enough energy input to ensure a penetration ratio of 100%, weld metal drips onto the rear side of the joint portion, leading to possible interference with the driving shaft and so on incorporated in the axle case 34. In extreme cases, a hole opens in the joint portion, leading to oil leaks and a reduction in the strength of the joint portion.
Hence, as shown in FIG. 11, a cylindrical strip 39 is inserted into the joint portion between the main body 38 and spindle 33. In so doing, weld metal can be prevented from dripping into the interior of the axle case 34 even when the penetration ratio is set at 100%.
However, demands have been made for further strengthening of the welded joint portions of the axle case 34 and so on.
For example, in the conventional welded portion constitution and welding method described above, a non-welded part 40 on the contact surface between the strip 39 and main body 38 and the strip 39 and spindle 33 may act as a preformed crack, causing stress to become concentrated in welding root portions 41 on the inner surface side of the welded portion. As a result, cracks may form from these points.
Moreover, contraction of the weld metal following welding may cause residual stress at the joint portion between the main body 38 and spindle 33. In this case, compressive stress in the longitudinal direction (the left/right direction in the drawing), which is beneficial in terms of strength, remains on the outer surface side of the joint portion, whereas tensile stress in the longitudinal direction, which is detrimental to strength, remains on the inner surface side of the joint portion.
In short, tensile stress remains on the inner surface side of the joint portion between the main body 38 and spindle 33 conventionally, and thus when an outside force is applied to the axle case 34, the stress becomes concentrated, leading to a possibility of cracking and the like.
In order to improve the strength of the welded joint portion between the main body 38 and the spindle 33, increasing the sheet thickness of the main body 38 and spindle 33 to increase the effective height of the welded joint portion has been considered, but in this case, the overall weight and cost of the axle case 34 rise.
Further, when the moment M (see FIG. 10) acts on the axle case 34 as described above, compressive stress is generated in the central portion of the upper member 30, whereas tensile stress is generated in the central portion of the lower member 31, and hence the circular hole formed by the joint portions therebetween deforms. As a result, a load which causes the cover member 32 and the upper and lower members 30, 31 to peel away from each other acts on these joint portions.
When a large load acts on the joint portion between the cover member 32 and the upper and lower members 30, 31 in this manner, stress concentrates in a part A (see FIG. 12) of the welded portion having the least strength, leading to the possible occurrence of cracking in this part. If a crack reaches the outer surface of the cover member 32, lubricating oil leaks out from the interior, causing a deficiency in the lubrication of the differential gear.
To prevent this, methods of increasing the sheet thickness t of the cover member 32 or providing an annular reinforcing sheet between the cover member 32 and the upper and lower members 30, 31 in order to increase the strength of the joint portion have been considered. However, these methods all lead to increases in the overall weight and manufacturing cost of the axle case 34, and are therefore unfavorable. Particularly in cases where a reinforcing sheet is provided, the number of components rises, and thus the number of production steps, assembly steps, and welding steps increases, leading to a large increase in manufacturing cost.